1. Field of the Invention
This invention relates generally to electric vehicles and more particularly to a differential turning control system for a vehicle having a pair of drive wheels and a steer wheel which pivots about a substantially vertical axis.
2. Related History
Electric motors have been employed as a traction motive source for various land vehicles including automobiles, trucks, delivery vans, wheel chairs, material handling trucks and industrial fork lifts.
A stored energy supply, carried by the vehicle, generally comprised batteries which accounted for a significant proportion of the total vehicle weight and, in the case of fork lift vehicles, provided a valuable counterbalance for the payload. Many fork lift vehicles included a pair of drive wheels positioned along a fixed common axis adjacent opposite sides of the vehicle. Each of the drive wheels was driven by a separate dc traction motor through a gearing.
During turning of the vehicle as opposed to straight movement, the inner drive wheel i.e. the wheel closest to the center of the curvature, traversed the support surface, e.g. cement warehouse floor, steel loading ramp, dock plate, etc., at a slower rate of speed than the outer drive wheel. In order to avoid slipping, erratic vehicle movement, loss of vehicle control, and undue drive wheel wear, it was therefore necessary to change the speed of the drive wheels relative to one another during turning.
In U.S. Pat. No. 3,870,935, issued to Ables et al., a system for permitting differential operation of industrial fork lift truck drive wheels was disclosed. The approach taken was to deenergize the traction motor connected to the inner drive wheel. Such deenergization was effected utilizing switches connected to a pulse generating circuit for selectively cutting off one or the other motor. This approach suffered from the disadvantage of rendering the vehicle difficult to maneuver and failed to provide a smooth transition in drive wheel speed variations as well as the requisite wheel speeds necessary for precise rolling contact between the drive wheels and the supporting surface during turns. Further, when one of the motors was turned off to effect a turning maneuver, the entire torque load was transferred to the other motor. Hence, each motor was required to have the size and capacity to individually propel the vehicle and load work. This resulted in high component costs associated with larger, more powerful traction motors than required when torque loads were shared between traction motors.
Maneuverability was an especially significant feature in connection with industrial fork lift vehicles which were often operated with limited visibility, in tight spaces, carrying heavy loads and with the load center elevated above the center of gravity of the vehicle.
Safety factors also required consideration of the maximum speed within which a turn having a specific radius of curvature could be safely effected, especially in view of variable parameters such as load weight range and load center elevation. Generally, as the radius of curvature of a turn decreased, maximum vehicle speed was required to be decreased.
In U.S. Pat. No. 4,520,299, issued to Konrad, a variable differential control system for a pair of drive wheel traction motors was shown. The system also implemented the constraint that maximum vehicle speed was to be limited as a function of a steer wheel angle. The control system employed a minimum field selector which assured a minimum traction motor field excitation; vehicle speed was not decreased until some minimum preselected turning angle was reached.
While the system disclosed in U.S. Pat. No. 4,520,299 presented a considerable improvement over the prior approach of merely turning off the traction motor for the inner drive wheel, disadvantages remained.
Prior control systems were also deficient because actual rotation speeds of the traction motors were not taken into consideration when demand signals were generated for differentially controlling the traction motor speeds to effect a desired steering maneuver. Instances occurred wherein, because of lag inherent in motor operation, one of the traction motors did not attain the speed necessary to effect the desired turn. Because the prior steering control systems separately excited each of the motors without monitoring the status of the motors, lag or error in actual motor speeds were not compensated, which may have resulted in uncontrolled operation.